Buzz has A LOT to say about this, and it's all correct. There's just so much to it!
On July 25, 2015 at 14:41, crosen said...
Suppose you have an amplifier that takes a standard, unbalanced, analogue line level audio signal via RCA terminals.
What I'd like to understand is what determines the maximum strength of this signal that the amplifier can handle before the amplification is clipped. Also, is this max oriented on voltage? or amps? or wattage? or ...?
First, the line input. A line input samples the input voltage. There's negligible current into a line input. The following amplifier stages amplify this signal.
Now, if there's an amplification stage right at this input, then the line input will cause distortion if, when amplified, the output level of that stage is too high of a voltage for that stage to cleanly pass. For instance, if that stage can output 3 volts but the line level is so high that, when amplified, 4 volts would come out of that stage, that last volt will not come through cleanly and will appear mostly as distortion.
Next, we have the output stages where voltage and current are delivered. If all the previous stages are clean but the signal presented by those stages to this output stage makes it try to output more voltage than it can do cleanly, then there's distortion.
There can also be distortion due to current limits. If the amp is connected to a low impedance load and the power supply voltage of the amp does not stay up at its normal voltage on peaks; then those peaks won't accurately represent the input signal. Not matching the input is the definition of distortion.
The question is not in connection with a particular project. It's just something I'd like to understand better to the end of better designied audio systems. Much appreciate any input.
There's a horrible term in this industry that is very helpful if you understand what it means and don't take it literally. That term is gain structure. If your gain structure is incorrect, you will have excess noise, excess distortion, or both.
Let's look at a system that's a variable output source; a preamp or mixer; and a trimmable power amplifier.
Let's say you have an adjustable output CD player (a pro model) in that system. If you turn the volume way down on the CD, you'll have to turn the volume way up on the mixer to hear it. Turning the mixer volume way up, you'll start to hear the hiss normal in all electronic circuits, that we mask by having the audio much louder than the hiss. Once this hiss can be heard, it's always there.
But if we turn up the CD player, we can then turn down the mixer, which will turn down the amount of hiss in the signal. See? Two components, different performance depending on where we set their volume controls.
Now, if the CD, at full volume, outputs so much volume that it causes distortion in the mixer, then you've got distortion and turning down the mixer won't fix it. Again, two components, and adjustments that can cause problems.
What you want to do is turn the CD up enough so that its level hides the noise but keep it down enough that it doesn't create distortion in the mixer. All this is called having proper gain structure.
Now add the power amp. If its volume controls are SET WAY THE HELL UP, then you might not be able to light more than one or two LEDs on the mixing board without driving people out of the place. And residual hiss in the mixer can be audible. Turn the power amp down some, you can turn up the volume control on the mixer, and things will work better. Gain structure is the balancing act of all inputs and outputs for minimum distortion and minimum noise.
The reason you can't take term "gain structure" literally? It's because every mixer, preamp, or power amplifier has gain blocks, circuits where a certain non-adjustable amount of gain occurs. Let's say we have a power amp with an input control and a maximum voltage gain* of 30. 1 volt of line level in equals an output voltage of 30. 30 volts AC is about 110 watts into 8 ohms.
What happens when we turn down the input control? We don't adjust the gain of the amp. It would still output 30 volts if fed 1 volt. We now feed it less than 1 volt, so its gain of 30 now outputs less than 30.
No volume control or slider adjusts the actual gain of any stage. It attenuates the signal from its input value all the way down to zero. And no gain ever changes.
However, it's convenient to talk as though we're changing the gain. To begin with, gain is only one syllable! Next, we're amplifying, so it's easier to talk about voltages increasing than talking about how we might minimize their decreasing. So we talk about changing gain as though that's what we're doing.
*going beyond talking about voltage gain is another subject
